TMDL Model Development for the Upper Kissimmee and Lake Istokpoga Basins

The Upper Kissimmee Basin and Lake Istokpoga Basin (Florida) have a combined area of 2,200 square miles (mi2). Both basins include areas of urban development, agriculture, and undeveloped area. Receiving waters in the basins include lakes, streams, and canals.Both basins have identified impaired waters with dissolved oxygen (DO) and nutrients as the parameters of impairment. In stream reaches, nutrient impairment is evaluated based on chlorophylla concentration, whereas lake nutrient impairment is based on the trophic state index (TSI), which is calculated as a function of lake concentrations of total phosphorus (TP), total nitrogen (TN), and chlorophyll-a. For a given lake, the target TSI value is based on the measurements of color in the lake.For both basins, the Hydrological Simulation Program—Fortran (HSPF) was used to develop hydrologic and water quality models that would be used to determine the TMDLs for the basins. For each basin, separate HSPF models were developed for the closed lakes and associated tributary areas (all in one model) and the open system of lakes, streams, and canals. In the Upper Kissimmee Basin, the receiving water system includes a chain of lakes with water control structures designed to maintain seasonal target water elevations. The depth-storage-outflow relationships in the FTABLEs for these lakes were designed to account for the seasonal control.The models were driven by NexRad precipitation from the state of Florida. Rainfall time series were developed for each model subbasin by averaging rainfall data for the rainfall grids associated with the subbasin. NexRad data were compared to rainfall gage data to validate the data, or to identify areas where the data were not reasonable (and replaced with more reasonable data).The hydrologic model calibration suggested that regional groundwater contributions were significant in some areas of the basins. These inflows were estimated using available groundwater elevation data and entered as point sources to the appropriate model subbasins, providing a better match between measured and modeled flows throughout the basins.The water quality calibration focused on generating reasonable land use-specific loads and calibrating the input parameters controlling the processes in the lakes. Statistical tests of measured and modeled means and distributions were used as the basis for the model calibration.The calibrated models were also run for undeveloped conditions, particularly to determine the expected TSI under these conditions. These results were used to assess whether the designated TSI target was appropriate or if a revised TSI target should be considered.